An example of a remarkable area where measurements of solid matter containing liquids is needed is forest industry, in which wood pulp samples or filtrates, such as e.g. wire water, white water, thickener filtrates or another similar pulp filtrate, or circulated water, need to be monitored in order to be able to control the overall process. E.g. in oil and mining industry processes and in water treatment industry, like water reuse, desalination processes and cooling water treatment, the liquids used often contain solid matter that need to be measured and monitored.
Such processes can be carried out off-line or on-line, where off-line methods often involve batch sampling and laboratory analyses. They have the benefit of providing accurate and versatile information on the suspension but suffer from considerable time delays. On-line methods, on the other hand, provide instant or almost instant information on the suspension, but the data that can be obtained is usually not as accurate as can be achieved in the laboratory. Some suspension properties cannot be measured using present on-line techniques.
Many such suspensions include particles, whose amount and size distribution have a considerable effect on upcoming process stages. E.g. agglomeration has, in fact, been shown to be the main threat for deposition and related running problems on paper machines. Liquids and filtrates in pulp industry also have a strong tendency to flocculate, which makes the analysis of the solid matter in liquid streams challenging.
Some prior art pulp sample or filtrate monitoring techniques have utilized sample fractionation e.g. by filtration, centrifugation, sedimentation or column flow. The only known continuous fractionator is a column flow fractionator, also called a “tube fractionator”. Tube fractionators are discussed e.g. in WO 2007/122289 and WO 2010/116030.
So-called flow cytometry technique has proved to be successful in detecting and assessing e.g. particle counts, size and/or type in pulp samples or filtrates originating from pulp and paper making industry. However, that technique requires manual sample pre-treatment in the laboratory and cannot be used for online measurements. Other known techniques discussed e.g. in WO 2012/010744 and WO 2012/010745 provide on-line information on the overall turbidity of samples. However, that information is not sufficient for all process control needs as the methods cannot differentiate different types of particles based, e.g. on hydrophobicity, particle size, and/or nature of the particles, whereby no detailed information is provided on disturbing substances.
Field flow fractionation (FFF) represents an approach in measurement of particles in non-industrial process samples. FFF was first described by J. C. Giddings in 1966 allows for physically separating particles having different physical properties from each other in a suspension. In principle, a flow of liquid is passed through a cell perpendicular to a field, e.g. a gravitation field, where smaller (lighter) particles move faster in the flow direction compared to larger (heavier) particles. Other fields that may be applied to the FFF cell include temperature and electricity.
In a flow cell, particles travel in a laminar flow and heavy particles sediment faster than light particles and therefore heavy particles experience extra friction upon touching the flow cell walls compared to light particles. There are many different FFF systems available depending on the application and most notably on the particle size range one wants to fractionate. For example, there are sedimentation FFF (SdFFF) systems available where the gravitational field is induced through centrifugal force.
It is however typical that an SdFFF system is only capable of handling very small quantities of sample, which is not applicable in a paper mill sample, if turbidity is used as the primary detector. The main problem with samples originating from industrial processes, e.g. with paper mill samples is the presence of fibers and especially fiber fines that have a strong tendency to flocculate in the FFF cell and thus block the cell. This makes the fractionation challenging as the flocks entrap also light particles.
In addition to flocculation, another problem is the mechanical or chemical sticking of substances to each other and attaching of stickies and hydrophobic substances to surfaces of known fractionation systems, in particular those based on cross-flow filters or known FFF techniques.
One technique for analyzing papermaking process samples is a method where harmful and uncontrolled agglomeration of pitch, stickies, scale, microbes and slime that disturb the papermaking process causing production down-time and paper defects are detected. The core of the system is the fractionation of particles according to their mass and/or size. The fractionated samples are analyzed with optical measurements.
The system is based on the Finnish patent application No. 20125560, filed by the present applicant, and is based on field flow fractionation, where the fractionating is performed by conducting the sample to a disintegration channel that one or more depressions, and by applying a liquid flow having a non-constant temporal velocity profile through the disintegration channel. In this way, solid matter of the sample will gradually be taken with the liquid flow from the depressions for providing sample fractions. This approach allows for measuring the particle size and/or mass distribution of a filtrate or a pulp sample and has proved to detect paper machine problems that cannot be seen with traditional measurements. There is no limitation as to the particle sizes that can be detected and measured, unlike many laboratory methods that work in the micron range.
The present invention seeks to further develop this and similar systems by developing a robust on-line system for continuous monitoring of hydrophobic/hydrophilic particles in water streams and pulp suspensions. Means of interpreting the results and to extract key variables for particle counts and hydrophobicity of a sample is also disclosed. Pre-treatment and separation of samples in order to achieve the objectives is described.